1. Field of the Invention
The present invention relates to integrated circuit packaging technology, and more particularly to flip chip integrated circuit package substrates.
2. Background Art
Integrated circuit (IC) chips or dies from semiconductor wafers are typically interfaced with other circuits using a package that can be attached to a printed circuit board (PCB). One such type of IC die package is a ball grid array (BGA) package. BGA packages provide for smaller footprints than many other package solutions available today. A BGA package has an array of solder ball pads located on a bottom external surface of a package substrate. Solder balls are attached to the solder ball pads. The solder balls are reflowed to attach the package to the PCB.
In some BGA packages, a die is attached to the substrate of the package (e.g., using an adhesive), and signals of the die are interfaced with electrical features (e.g., bond fingers) of the substrate using wire bonds. In such a BGA package, wire bonds are connected between signal pads/terminals of the die and electrical features of the substrate. In another type of BGA package, which may be referred to as a “flip chip package,” a die may be attached to the substrate of the package in a “flip chip” orientation. In such a BGA package, solder bumps are formed on the signal pads/terminals of the die, and the die is inverted (“flipped”) and attached to the substrate by reflowing the solder bumps so that they attach to corresponding pads on the surface of the substrate.
Flip chip package substrates are typically very expensive, for various reasons. A flip chip substrate requires a relatively high number of layers for an integrated circuit package substrate, causing costs to increase. The complexity of flip chip package substrates frequently leads to lengthy and expensive manufacturing processes for the substrates. Furthermore, non-recurring engineering (NRE) costs related to the design and manufacturing of flip chip substrates may be high. The advanced high density routing requirements for flip chip substrates typically lead to very expensive photolithography masks.
For BGA packages that include wire bonds, various functions of the die can be turned on/off through the use of wire bonds. For example, a first signal net of the BGA package can be activated by wire bonding a signal of the die to the first signal net. A second signal net may be deactivated by not wire bonding to the second signal net, or by wire bonding the second signal net to a deactivation voltage. Thus, wire bond packages offer some functional flexibility due to the use of wire bonds. Such functional flexibility enables some functional modifications to made to a wire bond BGA package without having to significantly alter the BGA package. However, flip chip packages do not provide such functional flexibility, because solder bumps of the die can only connect to pads of the substrate that are directly opposite the solder bumps. There is no ability to connect a solder bump of the die to an alternative substrate pad. Thus, when functional changes are made to a flip chip die, a new substrate may be needed to be fabricated for the flip chip package to enable the functional changes.
Thus, what is needed are ways of enabling functional flexibility in flip chip packages that do not require substantial modification of the flip chip packages when a change in functionality is desired.